Chief executive Paul Hodgkin said: “It is very difficult to see how these trusts are taking complaints seriously when they refuse to be open about their patients’ concerns.”

He warned the NHS would only be accountable if the public could see patient feedback about all trusts.

However, the South Warwickshire Foundation Trust, which runs the hospital, said it released complaints data in other, more detailed ways.

Three local NHS trusts reported a rise in complaints to the Department of Health during 2010/11.

They were NHS Warwickshire, George Eliot Hospital and UHCW, which runs University Hospital in Coventry and St Cross in Rugby.

Yet campaigners are more concerned about 29 foundation trusts which did not submit any figures. Mr Hodgkin, a former GP, said the incomplete figures could not be used to measure NHS performance.

Despite this, Foundations Trusts are not forced to submit complaints figures. Instead they can decide whether to do so themselves.

The NHS Information Centre urged all foundation trusts to report complaints. Yet this year the number refusing to do so rose from 18 to 29.

Katherine Murphy, chief executive, of the Patients’ Association, warned the trend undermined plans to make all hospitals foundation trusts.

She said: “We need to ensure there are the right safeguards in place. You can have local management, local involvement, local anything you want. But if local people do not have access to data to enable them to make comparisons it is a waste of time.”

Glen Burley, chief executive of South Warwickshire Foundation Trust, said not reporting complaints centrally helped to cut management costs as the government required.

The hospital reported complaints to the public in much more detail through its quality accounts, council of governors and public meetings of its board of directors, he said.